1. Field of the Invention
The present invention relates to laminated members on which an information code is recorded, articles to which the laminated members are attached, and methods for observing the information code, and more particularly, to an improvement of information reading performance obtained when an information code is formed on a transparent material layer of a laminated member.
2. Prior Art
Information codes such as bar codes and two-dimensional codes have been printed on components and on the packages of products for production management and distribution management. A well-known example is a point-of-sale (POS) information management system in which information codes are printed on products supplied to the market, and sales management and inventory management thereof are automatically performed with terminals in each shop and a host computer at a head office to which the terminals are connected. Two-dimensional codes can contain more information than one-dimensional bar codes, and can record many pieces of information such as production locations, production dates, and production numbers of products. To conduct complete production management and distribution management, it is necessary to use such information codes in this way.
General methods for writing such information codes in a metal surface or on a sheet include a laser marking method and an ink marking method. In the ink marking method, ink having a lightness (brightness) different from that of a base member is applied to a surface of the base member to write the code. In the laser marking method (using a YAG laser, a CO2 laser, or the like), a surface of a film or a metal is irradiated by a laser to form indentations on the surface to write the code.
As a method for reading the information code written in this way, the lightness difference (contrast) between the code and the base member is usually used. Various automatic code recognition units using a CCD camera have been developed. A code written by the ink marking method is read by using a lightness difference between the code written with the ink and the base member. A code written by the laser marking method is read by using a difference in the amount of light reflected from the indented portions and non-indented portions (see, for example, Japanese Patent No. 2719287).
As described above, to apply complete distribution management to products and to eliminate imitated products completely, it is necessary to directly print information codes on products or to directly attach to products sticky labels on which information codes are printed. Since the ink marking method requires large difference of a lightness between a base member and ink (black and white are the easiest to read combination), colors which can be used for the base member and ink are highly restricted. Therefore, the design of products deteriorates significantly when information codes are printed on or attached to the products.
To prevent the design from deteriorating, an indented-and-non-indented code is printed on a transparent film with the use of the laser marking method and the transparent film is attached to a product to show the information code without detracting from the design of the product.
However, an indented-and-non-indented code made by the laser marking method does not generate a sufficient lightness difference (contrast) between indented portions and non-indented portions on the transparent film. Therefore, reading the code may be impossible in automatic code recognition by the use of a CCD camera, or some reading errors may occur.
To solve this problem, for example, Japanese Patent No. 2719287, described above, has proposed a recognition method in which a surface where an indented-and-non-indented code is formed is irradiated with light at a certain angle, and reflected light obtained from an edge of a non-intented portion is received. This method, however, has some disadvantages: namely, the reading conditions should be specified in detail because the light illumination angle and the position of a light-receiving section are restricted, the apparatus structure becomes complicated, and the apparatus becomes expensive. The method also has a disadvantage in that the laser marking conditions should also be controlled in detail.